Spectral paper

From: Chris Lidman (clidman@eso.org)
Date: Fri Nov 19 2004 - 12:41:22 PST

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Hi Gabriele,
I've now revised the second part of the paper. We might want to go
through this process a couple of times before it is submitted to the
collaboration for review.

Cheers, Chris.

-- European Southern Observatory Alonso de Córdova 3107, Vitacura Casilla 19001, Santiago 19 CHILE

Ph. +56 2 463 3106 FAX +56 2 463 3001

text/x-tex attachment: Evolution_modfied_v1.2.tex__charset_UTF-8

General Comments
================

As before, I correct grammatical errors or I reword sentences. All my
changes are in bold.

On some occasions you use low-redshift and other occasions you use low
redshift, without the hyphen. You should choose one or the other but not
both.

Similarly, you might want to be a bit more consistent with the way
low-redshift SNe are descirbed. In the paper, you have used local,
nearby and low-redshfts as adjectives to describe the same thing.

Abstract
========

If the two SN1991T-like SNe in Matheson et al. have been correctly
identified, then the following is no longer true.

"making it the first object of this type identified at high reshift."

There are other similar sentences in the text which might have to
be removed or modified.

Introduction
============

The word "alternative" in the second paragraph implies that one alternative
has already been discussed, which is not the case. You can either drop
the word or move the discussion about lightcurve tests for homogeneity
from paragraph 3 to paragraph 1.

The following sentence seems to be incomplete.

"For example, attempts have been made to compare light curve parameters
but \citep{1999AJ....118.2675R,2000AJ....119.2110A,2001ApJ...558..359G}."

Section 2.2
===========

I removed the word "possible" from

"possible spectral peculiarities among SNe~Ia"

I do not understand comment "f" - "In Normal Ia typical line profile
time evolution."

Section 3.2
===========

Instead of

"Equivalent width ({\sc ew}) time evolution measurements have been
empirically shown to be excellent means to study the spectral homogeneity
of SNe~Ia and to provide reliable indicators of SN~Ia intrinsic brightness
\citep{folatelliew}."

I suggest

"Equivalent widths can be used to charactrise the spectral homogeneity
of SNe~Ia and to measure their intrinsic brightness\citep{folatelliew}."

I think that we should not use the word excellent in this sentence as
Gaston's paper has not been accepted (or even submitted). :(

---

In the second sentence, I replaced "signs of evolution" with "differences" since any observed diffrence may be due to either evolution or selection biases that affect low and high redshift SNe differently.

---

I added the sentence

"{\bf Since the {\sc ew}s of these features is a strong function of phase, the {\sc ew}s are plotted with respect to B-band maximum light.}"

---

In this and other parts of the paper, you should avoid the word evolution when describing the change in the EW with phase, beacuse readers may get confused between this and the evolution of SN~Ia properties with redshift. Instead, use the word trend.

---

I modified

"host galaxy contamination left after the host galaxy subtraction procedure as described in section \ref{data}."

to just "host galaxy contamination."

---

The word "break" in the first sentence of the second paragraph implies discontinuity. Instead of break, I've used the phrase "suddenly increases" that is in the second sentence, and I've re-arranged the first two sentences.

---

Change "sub-classes" to "sub-types"

Why was SN 2002gi not measured. Was the S/N ratio too low?

---

In some sentences you say under-luminous and in other sentences you say fast decliner. The reader may not be an expert, so it would be better to stick with one term.

You might need to describe inmore detail how the shaded region in figure 7 is defined.

In figures 5,6,7, you might need a sentence to explicitly state that the large symbols represent the high-z SNe.

Section 3.3 ===========

The following sentence appears out of place.

"The empirical models derived from local supernovae and high-redshift SNe data are plotted in Figs. \ref{fig:MgII} and \ref{fig:FeII}."

so I moved it up in the paragraph and reworded it.

Since the table describes the columns adequately, the following senetences are probably unecessary.

"The first two columns of Table~\ref{emdata} {\bf reports the $\chi^{2}$} refer to the comparison with the `Fe~{\sc ii} 4800' and `Mg~{\sc ii} 4300' {\sc ew} models for normal supernovae. The last two columns refer instead to the comparison with under-luminous SNe (e.g. SN~1986G \citep{1987PASP...99..592P} and SN~1991bg)."

I think that you need to give reasons why the mean value of the EW of the `Mg~{\sc ii} 4300' feature for high-redshift SNe~Ia is, on average, higher than the mean trend set by local SNe~Ia. Are we detecting some sing in the evolution, could it be a systematic effect caused by biases in the selection and/or analysis, or could it be that the sample size is just too small.

Table 3 =======

The following sentence is a bit vague.

"However, this assumes ideal conditions."

You should consider removing it.

---

I have two concerns about the following paragraph.

\citet{2000ApJ...530..966L} claim that the strength of supernova absorption features should be affected by the drift toward lower metallicity progenitor expected at high-redshift. However, measuring the intrinsic spread of {\sc ew}s of the absorption features of their models relative to those of the one solar metallicity synthetic spectrum, we found values lower than what is measured on the local supernovae. Thus, the range in which {\sc ew} vary -- studied in \citet{folatelliew} -- is dominated by other effects than their prediction from metallicity variations. The possible change of rest frame U-B color in high-redshift SNe is probably a more sensitive parameter to investigate the effects of varying metallicity.

How good are the models? Might it be that the models are not accurate enough. You dismiss metallicity as a possible cause for the measured intrinsic variation, because you measure a small variation in the models. If the models are not very accurate, then this might be not be the correct conclusion.

Since this paragraph discusses only the local SNe, perhaps it should appear in Gaston's paper. However, I have not seen any progress in Gaston's paper in the past six months, so I do not strongly believe that it should be removed. Nevertheless, you may want to modify it after considering my first point.

Section 3.4 ===========

Some of the numbers seem to be a bit precise.

114.06. 14.16. 68.69 6.08. 73.60 2.90

So I've rounded them to what I thought was more reasonable.

Section 4 =========

This section needs some improving.

In addition to summarising what was done in the paper, it might be useful if you write a paragraph describing the weaknesses and strength of the technique you have used. For example, obvious strengths are that the method is quantitative and can be applied to low and high redshift specrta and that the S/N of the high redshift spectra do not need to have very high S/N. A weakness is that the size of the systematic error instroduced by subtracting the light from the host galaxy is difficult to quantify and that high reshift SNe are likely to suffer more from this error that SNe at low redshift.

Also, I think you should add a paragraph on how the weakness could be overcome - e.g. a specrtrum of the host without the SNe.

The follwing sentence seems to say the same thing as the previous one.

"This classification was confirmed by means of the {\sc ew} measurements."

and then there is too much detail in the following sentences. This amount of detail should not appear in the summary.

"Prior to maximum light, Ca~{\sc ii}~H\&K {\sc ew}s show a clear distinction between the values typically found for normal and SN~1991T/SN1999aa-like SNe. The value measured for SN~2002fd ($ {\mbox{{\sc ew}}} $ = 73.60 $\pm$ 1.89) results consistent with that of SN~1991T/SN~1999aa-like object ($< {\mbox{{\sc ew}}} >$ = 68.69, $\sigma_{< {\mbox{\scriptsize {\sc ew}}} >}$ 6.08)."

Perhaps you want to summarize the comparison between low and high z first and the mention SN2002fd later

I don't think the last paragraph is necessary. It is not the main point of the paper, yet it is the final thing that is said.